Episodic layering of the early mantle by the ‘basalt barrier’ mechanism

Subducted oceanic crust is buoyant between depths of 660 km and about 750 km, and this causes early episodes of layering and breakdown in numerical models of the dynamical evolution of the mantle from 4.5 Ga ago until the present. During layered periods, the upper mantle cools and the lower mantle warms, reaching temperature differences of up to 300 °C, and oceanic crust is only a few kilometers thick, which would facilitate subduction and plate tectonics. Layering inhibits heat loss, so that high average mantle temperatures persist. The layering breaks down roughly every 100–150 Ma, at which time hot, fertile lower mantle floods the upper mantle and there is a dramatic burst of magmatism lasting a few million years and manifest in the models as oceanic crustal thicknesses of tens of kilometers. As radioactive heating declines and the mantle gradually cools, subducted plates eventually become thick and heavy enough to penetrate and disrupt the ‘basalt barrier’, and no further layering occurs after 1.6–1.8 Ga in the models. The models yield mean residence times of mantle material (between passages through melting zones) consistent with the mantle lead-isotopic apparent age of about 1.8 Ga. Accumulations of (denser) basaltic material at the base of the models preserve a remarkably clear record of the early magmatic pulses. These accumulations persist strongly into the present, probably because of high internal temperature and consequent low viscosity, even though they are unlikely to form under present conditions.